Electrochemical anodic oxidation of metals in suitable electrolytes is a widely used process for the formation of corrosion-inhibiting and/or decorative finishes on metals suitable for this purpose. These processes are briefly described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, volume 9 (1987), pp. 175-176. According to this reference, titanium, magnesium and aluminum and alloys thereof are anodizable, the anodization of aluminum and alloys thereof being of the greatest industrial significance. The electrolytically produced anodized coatings protect the aluminum surfaces from the action of weathering and other corrosive media. Anodized coatings are also applied in order to create a harder surface, thus increasing the wear resistance of aluminum. Particular decorative effects may be achieved by means of the intrinsic color of the anodized coatings or by absorptive or electrolytic coloring. Aluminum is anodized in an acidic electrolyte, sulfuric acid being most commonly used. Other suitable electrolytes are phosphoric acid, oxalic acid and chromic acid. The properties of the anodized coatings may be varied widely by selection of the electrolyte, the temperature thereof and by the current density and duration of anodization. Anodization is conventionally performed using direct current or using direct current having a superimposed alternating current.
Freshly anodized coatings may subsequently be colored by immersion in solutions of a suitable dye or by an alternating current treatment in an electrolyte containing a metal salt, preferably containing tin. As an alternative to subsequent coloring, colored anodized coatings may be obtained by so-called color anodization processes, in which anodization is performed in solutions of organic acids, such as in particular sulfophthalic acid or sulfanilic acid, each optionally mixed with sulphuric acid.
These anodically-produced protective coatings, the structure of which has been scientifically investigated (R. Kniep, P. Lamparter and S. Steeb: "Structure of Anodic Oxide Coatings on Aluminium", Angew. Chem. Adv. Mater. 101 (7), pp. 975-977 (1989)), are frequently described as "oxide coatings". The above investigation has, however, demonstrated that these coatings are vitreous and contain tetrahedrally-coordinated aluminum. Octahedrally-coordinated aluminum, as in aluminum oxides, was not found. Herein, the more general term "anodized coatings" is used instead of the misleading term "oxide coatings".
However, these coatings do not yet fulfill all requirements with regard to corrosion protection, as they still have a porous structure. It is consequently necessary to post-seal the anodized coatings. This post-sealing is frequently performed using hot or boiling water, alternatively using steam, and is described as "sealing". This treatment seals the pores, thus considerably increasing corrosion protection. There are numerous literature references relating to this post-sealing process. The following may be mentioned by way of example: S. Wernick, R. Pinner and P. G. Sheasby: "The Surface Treatment and Finishing of Aluminum and its Alloys" (volume 2, 5th edition, Chapter 11: "Sealing Anodic Oxide Coatings"), ASM International (Metals Park, Ohio, U.S.A.) and Finishing Publications Ltd. (Teddington, Middlesex, England) 1987.
However, not only are the pores sealed during post-sealing of the anodized coating, but a velvety deposit of a greater or lesser thickness, the so-called "sealing deposit", is formed over the entire surface. This deposit, which consists of hydrated aluminum oxide, is visually unattractive, reduces adhesion when bonding such aluminum components and promotes subsequent soiling and corrosion. Since the subsequent manual removal of this sealing deposit by mechanical or chemical methods is costly, attempts have been made to prevent the formation of this sealing deposit by means of chemical additives in the sealing bath. According to DE C-26 50 989, additions of cyclic polycarboxylic acids having 4 to 6 carboxyl groups per molecule, in particular cyclohexane hexacarboxylic acid, are suitable for this purpose. According to DE-A-38 20 650, certain phosphonic acids, for example 1-phosphonopropane-1,2,3-tricarboxylic acid, may also be used. The use of other phosphonic acids is known from EP-A-122 129. DE-C-22 11 553 describes a process for post-sealing anodic oxide coatings on aluminum and aluminum alloys in aqueous solutions containing phosphonic acids or salts thereof and calcium ions, wherein the molar ratio of calcium ions:phosphonic acid is adjusted to at least 2:1. A higher ratio of calcium ions:phosphonic acids of about 5:1 to about 500:1 is preferably used. Phosphonic acids which may, for example, be considered are: 1-hydroxy-propane-, 1-hydroxy-butane-, 1-hydroxy-pentane-, 1-hydroxy-hexane-1,1-diphosphonic acid together with 1-hydroxy-1-phenyl-methane-1,1-diphosphonic acid and preferably 1-hydroxy-ethane-1,1-diphosphonic acid, 1-amino-ethane-, 1-amino-1-phenyl-methane-, dimethylamino-ethane-, dimethylamino-butane-, diethylaminomethane-, propyl- and butyl-aminomethane-1,1 -diphosphonic acid, aminotrimethylene-phosphonic acid, ethylene-diamine-tetramethylene-phosphonic acid, diethylene-triamine-pentamethylene-phosphonic acid, aminotri-(2-propylene-2-phosphonic acid), phosphonosuccinic acid, 1-phosphono-1-methylsuccinic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid. On the basis of the practical examples of the said patent, this process is a conventional hot post-sealing process using post-sealing times of between 60 and 70 minutes at anodized coating thicknesses of between about 18 and about 22 .mu.m. Post-sealing time is thus approximately 3 minutes per .mu.m of coating thickness.
When using water which contains no additives other than the stated sealing deposit inhibitors, elevated temperatures (at least 90.degree. C.) and relatively long treatment times of the order of about 1 hour for an anodized coating of about 20 .mu.m have hitherto been necessary. This corresponds to a post-sealing time of about 3 minutes per micrometer of anodized coating thickness. The post-sealing process is thus highly energy intensive and, due to its duration, may act as a bottleneck in the production process. Attempts have thus already been made to find additives for the post-sealing bath which promote the post-sealing process, so that it may proceed at lower temperatures (so-called cold post-sealing or cold sealing) and/or using shorter treatment times. The following have, for example, been proposed as additives which facilitate post-sealing at temperatures of below 90.degree. C.: nickel-salts, in particular fluorides, which are sometimes used in practice (EP 171 799); nitrosyl-pentacyanoferrate; complex fluorides of titanium and zirconium together with chromates or chromic acid, optionally in conjunction with further additives. As an alternative to actual post-sealing, hydrophobization of the oxide coating by means of long-chain carboxylic acids or waxes has been recommended, as has treatment with acrylamides, which should apparently be polymerized in the pore voids. Further details in this connection may be found in the above-mentioned reference by S. Wernick et al. With the exception of post-sealing using nickel compounds, it has not proved possible to implement these proposals in practice.
Processes for cold post-sealing using nickel fluoride have been implemented industrially, However, due to the toxic properties of nickel salts, this entails costly waste water treatment measures.
There is thus still a need for alternative post-sealing processes for anodized surfaces which make it possible to increase the rate of production by shortened post-sealing times and/or to reduce the energy consumption necessary for post-sealing, without using heavy metals, such as nickel, which are questionable on environmental and health grounds.
An accelerated, hot post-sealing process is known from U.S. Pat. No. 5,411,607 in which the anodized metal components are immersed in an aqueous solution containing lithium. The lithium concentration is preferably from 0.01 to 50 g/l, in particular from 0.01 to 5 g/l. It is moreover suggested that the sealing solution should additionally contain a sealing deposit inhibitor. This is preferably present in a concentration of between 0.1 and 10 g/l and is preferably an aromatic disulfonate. According to U.S. Pat. No. 5,478,415, which has the same priority as the above-mentioned U.S. Pat. No. 5,411,607, accelerated hot post-sealing may proceed using an aqueous solution which contains at least 0.01 g/l of lithium ions and 0.1 to 10 g/l of a sealing deposit inhibitor. Here too, the sealing deposit inhibitor is preferably an aromatic disulfonate.
German patent application 195 38 777.5 discloses an accelerated hot post-sealing process in which the anodized metal components are contacted with an anodizing solution which contains a total of 0.1 to 5 g/l of one or more alkali metal and/or alkaline earth metal ions and a total of 0.0005 to 0.2 g/l of a sealing deposit inhibitor in the form of phosphonic acids or cyclic polycarboxylic acids.
The teachings of the latter three cited documents allow hot post-sealing times to be shortened substantially. It would, however, be desirable on economic and environmental grounds to have post-sealing processes available which consume distinctly smaller quantities of chemicals. An object of the present invention is to provide such a process.